1. Field of the Invention
The present invention relates to a high-performance SCR catalyst and β-zeolite used therein.
2. Description of Related Art
β-zeolite is known as a zeolite used in NOx reduction catalysts, and particularly NOx reduction catalysts using ammonia as a reducing agent (typically referred to as selective catalytic reduction (SCR) catalysts) (see Patent Document 1).
SCR catalysts using β-zeolite were unable to be used following durability treatment in a high-temperature steam atmosphere (to be referred to as “hydrothermal aging”) due to a decrease in NOx reduction performance, and due in particular to a considerable decrease in performance at temperatures of 300° C. or lower. Although this decrease in performance of SCR catalysts using β-zeolite following hydrothermal aging is thought to be mainly caused by insufficient heat resistance of β-zeolite, the cause of the considerable decrease in performance at low temperatures in particular has yet to be adequately determined. The performance of SCR catalysts using β-zeolite reported in previous documents or able to be acquired commercially is such that the NOx reduction rate is no more than 30% at 200° C. following hydrothermal aging.
β-zeolite is widely known to be used in catalysts or adsorbents, and methods such as increasing the silica/alumina ratio or increasing crystal diameter are known for improving the heat resistance thereof (see Patent Documents 2 and 3). However, in the case of increasing the silica/alumina ratio or increasing crystal diameter, the performance of SCR catalysts, and particularly the performance thereof at low temperature, has been unable to be improved.
Several proposals have been made in the past regarding improvement of the performance of SCR catalysts using β-zeolite. For example, a method for carrying out dealumination treatment by treating with steam and heat prior to use has been proposed (Patent Document 4). However, performance following hydrothermal aging was still not adequate. In addition, a method has also been reported for improving heat resistance by adding a rare earth element in addition to iron ordinarily used as a supported metal for SCR catalysts (Patent Document 5). However, the NOx reduction rate at 200° C. is still less than 30% even by using this method, while also requiring an expensive rare earth element.
In addition, since ammonia is used as a reducing agent in conventional SCR catalysts, the problem of unreacted ammonia being discharged resulting in the discharge of irritating, toxic gas has yet to be resolved (Patent Document 5).
Various methods have been proposed thus far for the production of β-zeolite (Patent Documents 5 to 9). However, β-zeolite having a high NOx reduction rate following hydrothermal aging, and particular β-zeolite having a high NOx reduction rate at low temperatures, has yet to be proposed for use as β-zeolite used as an SCR catalyst.    [Patent Document 1] Japanese Patent No. 2904862    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H9-38485    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. H11-228128    [Patent Document 4] Published Japanese translation No. 2004-536756 of PCT International Publication    [Patent Document 5] Japanese Laid-Open Patent Application, First Publication No. 2005-177570    [Patent Document 6] Japanese Unexamined Patent Application, First Publication No. S61-136910    [Patent Document 7] Japanese Unexamined Patent Application, First Publication No. S61-281015    [Patent Document 8] Japanese Unexamined Patent Application, First Publication No. H5-201722    [Patent Document 9] Japanese Unexamined Patent Application, First Publication No. H6-287015    [Patent Document 10] Japanese Unexamined Patent Application, First Publication No. H7-247114    [Patent Document 11] Japanese Unexamined Patent Application, First Publication No. H9-175818